[NiL2]C4O4. nH(2)O [where n=2 when L=diethylenetriamine(dien) and N-(3-aminopropyl)-1,3-propanediamine (dpt); n=3 when L=N-(2-aminoethyl)-1,3-propanediamine (aepn); n=0 when L=N-2-methyldiethylenetriamine (medien)] and Ni(tmdien)C4O4. 2H(2)O (where tmdien= 1,4,7 trimethyl-diethylenetriamine) have been synthesised and investigated thermally in the solid state. Ni(dpt)C4O4. H2O has also been synthesised pyrolytically in the solid state from the corresponding bis complex. All the complexes possess octahedral geometry. The squarate anion takes part in coordination only in monotriamine species. [Ni(medien)(2)]C4O4 upon heating undergoes phase transition (270-285 degrees C; Delta H=4.9 kJ mol(-1)) accompanied by colour change pink to grey. Thermal stability decreases with increase in chain length of the triamines.